A lean burn engine is characterized by being fuel efficient, but a normal three-way catalyst cannot purify NOX in a lean air-fuel ratio region (oxygen excess atmosphere). An NOX storage reducing catalyst developed with a new purification principle is able to purify NOX in the lean region and comprises a normal three-way catalyst with an additional alkali metal, alkali earth metal, etc. as material that holds NOX (storing/adsorbing material). Under normal operation at a lean air-fuel ratio, NOX is oxidized by excess oxygen on the catalyst precious metal to become NO2 and is held on the NOX holding material as nitrates etc. Further, by briefly making the air-fuel ratio rich (fuel excess atmosphere), the nitrates etc. are reduced by CO and HC on the catalyst precious metal to be purified and expelled as N2.
An NOX storage reducing catalyst is produced by forming an alumina or other catalyst support layer (coat layer) on cordierite or another base material and making this carry Pt or another catalyst precious metal and K or another NOX holding material. Generally, a catalyst support layer is formed by adding titania (TiO2) to alumina (Al2O3), zirconia (ZrO2), ceria (CeO2), and other such porous support particles so as to suppress the sulfur poisoning distinctive to NOX storage reducing catalysts. The presence of titania promotes the desorption of SOX held by the NOX holding material when regenerating the catalyst from sulfur poisoning in a high temperature rich environment. However, there was the problem that the addition of titania caused a large drop in the heat resistance of the catalyst.
WO00/00283 discloses a NOX storage reducing catalyst using a support comprising titania and zirconia carrying Rh in advance. It is considered that titania suppresses sulfur poisoning and carrying Rh on ZrO2 improves the purification capability. However, the heat resistance of titania is low, so both the titania itself and the catalyst precious metal easily sinter and the expected improvement in the purification capability cannot be acquired.
Japanese Patent Publication (A) No. 2001-9279 discloses an NOX storage reducing catalyst using a support comprising alumina particles with surfaces coated by titania particles of particle sizes of 10 nm or less. Further, as a method of producing the same, this discloses to make the pH of a slurry comprised of the alumina particles and titania sol less than 5, then increase the pH to thereby coat the alumina particle surfaces with titania fine particles. However, with the method of production of Japanese Patent Publication (A) No. 2001-9279, as shown by FIG. 10 of Japanese Patent Publication (A) No. 2001-9279, the titania sol passes the isoelectric point over the course of increasing the pH, so agglomerates and immediately coarsens at that point. With the pH at this time, the alumina becomes charged and does not agglomerate with the titania sol. Further, when increasing the pH, the titania sol agglomerates begin to become charged negatively. However, in comparison to agglomeration, redispersal requires a long time, so the titania particles are adsorbed on the alumina in the agglomerated and coarsened state. In this way, with the method of production of Japanese Patent Publication (A) No. 2001-9279, there is a high likelihood of the result being not the state with the surfaces of the alumina particles being covered with titania fine particles, but the result with the alumina particle and agglomerated titania particles simply being mixed together.
Japanese Patent Publication (A) No. 2004-321847 discloses an NOX storage reducing catalyst using a support obtained by calcining an alumina/titania composite oxide prepared by a coprecipitation method carrying a titania precursor. However, titania has a low heat resistance, so a sufficient heat resistance is not acquired.
Further, even in a three-way catalyst, sulfur deposits on the catalyst support as sulfates, so there is the smell of H2S generated under idling conditions etc. after high speed driving.
In this way, if adding titania to a catalyst support, the heat resistance of the catalyst drops in comparison to the case where no titania is added, so the effect of suppression of sulfur poisoning brought about by addition of titania and the purification capability inherent to the catalyst cannot both be achieved.